Instead of heat treating an older bow that took too much set, take the experiences learned from that and other bows and put that positive energy towards building a more successful bow. I have used heat treating(tempering) to reduce set and/or increase compression strength in bows that took too much set and it works to some extent. You would be better served by heat treating and adding a limited amount of backset(reflex) to a bow being built so not to damage the belly cells to begin with. Adding too much reflex can also cause excessive compression stress in a bow being built so you end up with the same situation as you did with a bow with too much set. By adding 2" to 3" of backset and tempering the belly(primarily with whitewoods) as you straighten(laterally) your stave you can achieve a bow with 1" of reflex or a flat profile that will maintain it's shape for the rest of it's life, which can be many thousands of shots.
I have used tempering on osage with good results but generally osage is compressing strong enough without it. If you take your time while building a wood bow, don't over stress the wood, don't pull it beyond its ultimate draw weight of length and exercise the limbs with each wood removal you can build a wood with a limited amount of set. Instead of putting your energy into trying to fix a bow with too much set, figure out why it has too much set and work towards eliminating or minimalizing the stresses that caused that set to begin with. Over stressing bow wood is the main cause of set and I am still guilty of doing this after building hundreds of bows over the last 25 or so years. If your mind and concentration aren't into the building process, put down the tools and walk away until you can concentrate on a positive bow building processes.